# Why do rockets have to hit the atmosphere at an angle on reentry to not burn up?

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I remember this from Apollo 13, they had to hit the atmosphere at an angle, if they came in too directly they’d burn up. My stupid layman thought is that I’d want to come in directly because if the atmosphere is making me burn up I’d want to take the directest and shortest route to landing so that there’s less atmosphere to burn me up. Obviously that’s not how it works, why not

In: Planetary Science

The atmosphere gets thinner as you go up. Less air means less burning up. It also means less force on the craft and more time to slow down.

So you design your ship around a certain descent profile, balancing these needs and more. Then, if you go outside of the parameters you designed for, shit explodes.

Air surrounds the Earth. When you press into it, it compresses. As it compresses, it gets hot. If you press into it fast enough you compress it enough that it heats up to a temperature hot enough to melt rock. That’s what’s happening in a meteor (shooting star).

Astronauts don’t want their spacecraft to melt, that’s bad (see Space Shuttle Columbia disaster). At a shallow angle you get hot for longer, but not hot enough to melt. That’s what the spacecraft is going for. Too shallow and you skip off, like a rock skipping on a pond.

If an lander attempted to come in at a steep angle they WILL burn up There is a lot more atmosphere to burn you up at lower altitude than there is at high altitude

It is not that our upper atmosphere is super hot, the heat is from friction between our atmosphere and the lander. When the lander is in low earth orbit they are going 24,840-27,772 km/h or 15,435-17,224 mph

So when the lander starts re-entry it come in contact with the atmosphere and there is a LOT friction this does 2 things

Creates a lot of heat

Slows down the lander

this is super important as you need to slow down alot before you can safely get deeper into the atmosphere

TLDR: it is the speed of the lander that causes the heat not the fact that it is in the upper atmosphere

A heat shield on a re-entering spacecraft has to contend with heat in two different forms: peak heating and total heating. Coming straight back into the atmosphere decreases total heating, but increases peak heating. Taking a long gentle angled path across the edge of the atmosphere increases total heating, but decreases peak heating. You need a happy medium for both.

As a re-entering object rips through the atmosphere, it compresses the air to the point that the air starts to heat to the point of glowing (this is what you see when you see a shooting star), and the glowing air radiates heat into the object. That heat has to go somewhere, or the spacecraft will melt. Most of the human space capsules that have re-entered use ablative heat shields — cork or some other carbon-based shield with a surface that burns off gradually to carry away the heat. If your path through the atmosphere is too long, you don’t slow down particularly fast and so you remain at ultrasonic velocities for a long time, which means that your heat shield might burn away completely. That would be bad. But if you dive straight down into the atmosphere, you hit the very dense atmosphere much more quickly, and so the air gets MUCH hotter, hot enough that your heat shield cannot carry away heat quickly enough and so it fails. That would also be bad.

The Space Shuttle and the SpaceX Starship (and the X-37) use radiative tiles instead of ablative heat shields. Radiative tiles are able to get rid of waste heat by simply radiating it away, which works very well; they don’t get “used up” at all. But radiative tiles can’t handle the same level of peak heating that an ablative heat shield can manage, so the Shuttle and Starship have to use a much more gradual trajectory. Spending more time on re-entry isn’t a problem for these vehicles since the heat shield isn’t burning away and thus won’t get used up.

The Shuttle also had the additional problem of aerodynamics; it had to be able to maintain aerodynamic control during re-entry while also being able to fly well enough to land on a runway. So it had to do a bunch of additional things to maintain the correct trajectory.

At a shallow angle allows them to pass through thinner atmosphere for a longer time to lose kinetic energy more gradually. This allows for lower force on the craft and more time for heat dissipation.

Coming straight in dumps force and energy into the craft aggressively, and the extreme forces and heat will either break or melt (actually ionize into plasma) the craft.